1. Field of the Invention
The present invention relates to air intake systems for internal combustion engines, and, more particularly, to an air intake system which is adapted to establish and maintain a predetermined combustion air temperature by means of a flow proportioning valve which admits varying quantities of cold raw air and preheated raw air into the air intake system.
2. Description of the Prior Art
The temperature of the combustion air of an internal combustion engine affects not only its efficiency as an energy converter and its overall energy output, but also the completeness of combustion and, consequently, the emission of pollutants by the engine. Unfortunately, the optimal air temperature, for a maximum power output, is not the same as the optimal temperature for the minimization of pollutant production. The latter calls for a relatively high air temperature, in order to produce a better fuel-air mixture through more rapid vaporization of the fuel, whereas for a maximum power output at full load, it is desirable to use relatively cold raw air, in order to improve the volumetric efficiency of the engine; colder air being denser than warm air.
It has been found that these conflicting objectives are best served by adjusting the combustion temperature to a higher level for partial-load operation of the engine and to s lower level for full-load operation. Ideally, these temperature levels should be in the neighborhood of 30.degree. C. and 10.degree. C. The preheating of cold raw air during the warmup portion of engine operation has the additional advantage of accelerating the warmup process and of minimizing operation in a cold state in which the moving parts of the engine are poorly lubricated.
While it is not practical to lower the combustion air temperature through cooling means, if the ambient temperature is higher than the optimal temperature level, it is relatively easy to raise the air temperature from a low ambient temperature level, by using the exhaust system of the engine as a heat source. The prior art in this field contains a number of suggestions for supplying preheated raw air to the intake system of the internal combustion engine, most of them featuring a warm air intake duct which is connected to a so-called exhaust stove and which feeds warm air into a duct junction in the main intake duct which leads to the air intake filter of the engine. In the duct junction is arranged an adjustable air flow proportioning valve which, in one end position, closes the warm air intake duct while allowing a maximum flow through the cold air intake duct, and, as it progressively opens the warm air intake duct, restricts the flow from the cold air intake duct, until only preheated raw air enters the air intake system.
A favorite solution involves the arrangement of the duct junction and air flow proportioning valve in such a way that the warm air intake duct joins the larger horizontal cold air intake duct from below, forming a valve seat for a pivotable flapper valve. The flapper, when oriented horizontally, closes the warm air intake duct and, when moved to an upwardly inclined orientation, opens the warm air intake duct, while substantially closing the cold air intake duct. Various drive means are used to control the angular position of the flapper, in order to achieve the desired air mixture and temperature adjustment.
The German Pat. No. 20 17 983 discloses a flapper valve which is connected to the drive rod of a longitudinally expanding and contracting wax thermostat. This thermostat may be arranged either inside the air intake duct, downstream of the duct junction, or on the outside of the intake duct system. In each case, the wax thermostat moves its drive rod in response to a heating coil which is energized electrically, in accordance with a temperature signal received from a sensor in the air intake system, upstream of the carburetor. The temperature signal is amplified and converted by a signal transformer. This prior art solution has the shortcoming that its response to temperature changes is slow and that it does not make adjustments for different temperature levels for partial-load operation and full-load operation.
A different solution is disclosed in U.S. Pat. No. 3,726,512 which suggests a pneumatic membrane actuator connected to the pivotable flapper valve, the actuator receiving negative pressure from the engine intake manifold, downstream of the carburetor throttle, the negative pressure being adjustably reduced by means of a thermostatically controlled relief valve. The latter is arranged inside the clean air space of the air intake filter, upstream of the carburetor, so that its thermostatic member is responsive to the temperature of the incoming combustion air. An increase in this temperature progressively opens an air intake passage in the relief valve, so as to reduce or eliminate the negative pressure which is transmitted to the membrane actuator. A return spring in the actuator then extends the actuator drive rod to move the flapper valve towards the position in which it closes the warm air intake duct.
Like the previously described device, this prior art solution reacts to temperature changes with considerable inertia, tending also to overshoot the equilibrium position. However, the use of negative pressure from the engine intake manifold as a drive medium offers the possibility of achieving a lower combustion air temperature under full-load operation, because, with a fully open carburetor throttle, the engine intake does not generate enough negative pressure in the drive system to move the flapper away from its rest position in which the warm air intake duct is closed, whether the ambient air temperature is high or low.